1. Field of the Invention
The present invention relates to a magnetic recording medium.
2. Description of the Related Art
A hard disk drive that is one of magnetic recording devices with floating of a record converting element (or a head) that is an information recording unit, is widely used as an external storing unit for computers and other various information terminals, in general.
Current magnetic disks employ, as a magnetic layer, a thin magnetic alloy layer made of a cobalt alloy indicating good magnetic properties on a hard nonmagnetic substrate. However, the magnetic alloy layer is significantly inferior in durability and corrosion resistance, and tends to cause degradation of magnetic properties and mechanical or chemical damage, owing to friction through contact with or sliding on a head, and corrosion generated by the wear and moisture adsorption. Accordingly, as matters now stand, a protective layer is laid over the surface of the magnetic alloy layer, and a lubricant layer made of a lubricant is laid on the protective layer so as to improve the durability and corrosion resistance.
As the protective layer, various materials such as SiO2 and Al2O3 are used. However, it is presently considered that a carbon-type protective layer comprising carbon such as an amorphous carbon is desirable as a protective layer for a magnetic recording medium as well as a head, in terms of thermal stability, corrosion resistance and wear resistance. Carbon-type protective layers deposited by the sputtering method and the CVD (Chemical Vapor Deposition) method are generally used.
In the current information-intensive society, the amount of information to be handled shows a tendency to increase in every application. Accordingly, further increase of recording density and storage volume is anxiously anticipated for magnetic recording discs. In order to meet the requirement for a higher recording density, it is indispensable to shorten the distance between the magnetic layer and the information recording/reading part of the head, that is, so-called magnetic spacing, and therefore, it is believed necessary to make thinner the protective layer itself.
Accordingly, there has been increasing a strong need for a protective layer that can secure a sufficient durability even if it is as extremely thin as 5 nm or less. In recent years, the Filtered Cathodic Arc method (the FCA method) has drawn great attention by which it is possible to form a protective layer having better durability than a conventional protective layer (see IEEE TRANSACTIONS ON MAGNETICS, H. Hyodo, et al., vol. 37. p. 1789-1791, July 2001).
An amorphous carbon can be prepared by the FCA method as follows. FIG. 3 is a schematic view of a FCA deposition system to perform the FCA method. In reference to FIG. 3, a carbon source such as graphite is used as a cathode 31. Arc discharging is caused between the cathode 31 and an anode 32 to generate carbon ions, electrons, carbon neural atoms and carbon macroparticles, from which the carbon neutral atoms and carbon macroparticles are removed by magnetic filters (filter coils 33 and 34) so that only the carbon ions and electrons are sent to a substrate 35. Thus, a DLC (diamond-like carbon) layer 36 is formed on the substrate. An ion gun 37 is used for doping with other elements.
With the FCA method, it is easy, based on the deposition principle, to increase the amount of sp3 bonding that is generally called diamond bonding, to 50% or more of the total amount. Accordingly, it is possible to realize a hardness and density similar to those of diamond in an amorphous form. However, a protective layer formed by the FCA method has a disadvantage that it is liable to be exfoliated from the magnetic layer to be protected, owing to its large internal stress.
It has been also found that though a protective layer formed by the FCA method has a high coating capability as a film, it is inferior in corrosion resistance to protective layers formed by the conventional methods, and the inferiority becomes more significant when it is thinner. It is considered that it is caused indirectly by the high internal stress that the FCA film has. That is, it is supposed that damage of the protective layer owing to the release of the stress occurs at an interface of the protective layer and a magnetic layer that has undergone corrosion, with the result that corrosion at the exfoliated surface of the protective layer is accelerated.
Furthermore, though a protective layer formed by the FCA method has a high coating capability as a film, there is a problem that corrosion of the magnetic layer occurs, caused by the moisture that has intruded into the magnetic layer through small defected parts such as pin holes, when the protective layer is as extremely thin as 5 nm or less. Such a problem cannot be ignored.
Because of the present status as described above, it is difficult to readily apply an amorphous carbon protective layer formed by the FCA method from the viewpoint of a protective layer for the magnetic recording medium, though it is surely excellent in mechanical strength.
It is an object of the present invention to solve these problems, and to provide a magnetic recording medium having excellent durability and corrosion resistance as well as a magnetic recording device equipped with the magnetic recording medium. Other objects and advantages of the present invention will be clarified through the following explanation.